The invention relates to an apparatus for the examination of specimen with a beam of charged particles. In particular, this invention relates to an objective lens for a charged particle beam device.
The resolution of the conventional optical microscopy is limited by the wavelength of the visible light. Furthermore, at the highest resolution the conventional optical microscopy has a very shallow depth of field. These two limitations have led to the increased popularity of charged particle devices for the examination of specimen. Compared to optical light, accelerated charged particles, for example electrons, exhibit a shorter wavelength, which leads to an increased resolution power. Furthermore, even at the highest resolution charged particle devices usually exhibit a large depth of field. Accordingly, charged particle beams, especially electron beams, are used in a variety of ways in biology, medicine, the materials sciences, and lithography. Examples include the diagnosis of human, animal, and plant diseases, visualization of sub cellular components and structures such as DNA, determination of the structure of composite materials, thin films, and ceramics, or the inspection of masks and wafers used in semiconductor technology. In addition to that, charged particle beam devices, may also be used for the modification of organic and inorganic materials and their surfaces.
In these instruments, the area to be examined and/or modified is irradiated with a charged particle beam, which may be static or swept in a raster across the surface of the specimen. Depending on the specific application, the charged particle beam is more or less focused and the kinetic energy of the particles can vary considerably. The types of signals produced when the charged particles impinge on a specimen surface include e.g. secondary electrons, backscattered electrons, Auger electrons, characteristic x-rays, and photons of various energies. These signals are obtained from specific emission volumes within the sample and can be used to examine many characteristics of the sample such as composition, surface topography, crystallography, etc.
In charged particle beam devices, such as a scanning electron microscope (SEM), the charged particle beam exhibits a typical aperture angle as well as a typical angle of incidence in the order of several millirads. However, for many applications it is desirable that the charged particle beam hits the sample surface under a much larger angle of typically 5xc2x0 to 10xc2x0, corresponding to 90 to 180 millirads. Stereoscopic visualization is an example for such an application. Some applications even require tilt angles in excess of 15xc2x0 or even 20xc2x0. Thereby, a number of tilting mechanism can be used. In early solutions, an oblique angle of incidence was achieved by mechanically tilting the specimen. However, due to mechanical imperfections, a lateral movement of the specimen is inevitable, which often results in misregistrations between two pictures having two different viewing angles.
An oblique angle of incidence may also be achieved by electrically tilting the charged particle beam. This is usually done by deflecting the beam so that either by the deflection alone or in combination with the focussing of the beam an oblique angle of incidence results. Thereby, the specimen can remain horizontal, which is a significant advantage as far as the lateral coordinate registration is concerned. Electrical tilting is also much faster than its mechanical counterpart. The electrical method, however, has also certain drawbacks. Especially, in low energy electron microscopy the magnetic or compound objective lens has to be very strong with very short focal length (1-20 mm), in order to achieve high resolution. In the presence of such a lens it is difficult to influence the landing position or landing angle of the charged particle beam at the specimen surface. In general, the strength of the deflector field has to be comparable to the field of the objective lens. To achieve such strong deflection fields the deflector usually has to employ pole pieces made from magnetically soft material (e.g. mumetal or permenorm). Furthermore, it is usually necessary to concentrate the deflection field to an area close to the optical axis. However, in conventional systems the corresponding pole pieces can not be placed close to the objective lens gap because they would negatively influence the field distribution of the lens. If, however, the deflector is placed in sufficient distance after the objective lens, usually a resolution degradation due to the increased working distance will result. Furthermore, if the deflector is placed before the objective lens, usually high offaxis aberrations will result.
The present invention provides an improved objective lens for a charged particle beam device. According to one aspect of the present invention, there is provided a objective lens for a charged particle beam device as specified in independent claim 1. According to a further aspect of the present invention there is provided a column for a charged particle beam device as specified in independent claims 16 or 17. Further advantages, features, aspects and details of the invention are evident from the dependent claims, the description and the accompanying drawings. The claims are intended to be understood as a first non-limiting approach of defining the invention in general terms.
The present invention provides an improved objective lens for a charged particle beam device. The objective lens comprises a magnetic lens that creates a first magnetic field for focussing the charged particle beam onto the specimen. Furthermore, a deflector is integrated into the magnetic lens by providing at least one additional coil arrangement that creates a second magnetic field used to deflect the charged particle beam. Thereby, the second magnetic field is guided through at least one of the pole pieces of the magnetic lens. The present invention also provides an improved column for a charged particle beam device including the improved objective lens.
By integrating the deflector into the objective lens, large angles of incidence can be achieved without causing large lateral movements of the charged particle beam on the specimen. Furthermore, due to the integration of the deflector into the objective lens, the working distance of the system can be kept small, so that the resolution of the system is not negatively influenced. The objective lens can be used to produce stereo images of a specimen in a fast and reliable manner. Accordingly, the additional information which is contained in stereo images and which is extremely valuable in many cases, can be accessed without causing any additional costs.
The present invention also provides an improved column for a charged particle beam device. The column comprises a magnetic deflector having at least one excitation coil arrangement for generating a magnetic field to deflect the charged particle beam and being arranged between the objective lens and specimen whereby the objective lens concentrates the magnetic field of the deflector in a region close to the specimen Due to field termination effect of the objective lens, the magnetic deflector can be placed very close to objective lens without interfering with the focussing properties of the objective lens. Accordingly, the working distance of the system can be kept small, so that the resolution of the system is not negatively influenced